Alteration in the Mechanical Properties of Human Ovum Zona Pellucida Following Fertilization: Experimental and Analytical Studies

The unfertilized oocyte is surrounded by a spherical layer called the zona pellucida (ZP). The physical hardness of this layer plays a crucial role in fertilization and it is largely unknown because of the lack of appropriate measuring and modeling methods. Recently, considerable biomedical attentions have concentrated on determination of the mechanical properties of oocytes as a single cell. In order to investigate the biophysical characteristics of mammalian oocytes, a change in the elasticity of human ZP has been quantitatively evaluated before and after fertilization. Young's modulus of ZP of metaphase-II (MII) and pronuclear (PN) stages have been estimated using two different protocols of the micropipette aspiration, step-by-step and continuous increase in pressure, in combination with proportional theoretical models. Experimental results clearly demonstrated that after fertilization the mean Young's modulus of the ZP calculated from the step-by-step aspiration test (MII: 7.34 ± 1.36 kPa vs PN: 13.18 ± 1.17 kPa.) and continuous aspiration test (MII: 2.41 ± 0.75 kPa vs. PN: 4.43 ± 1.66 kPa) significantly increased, (p < 0.05). Mathematical Evaluation of the results shows that although the results of the two methods are different but both confirm that the hardening of ZP will increase following fertilization. As can be seen, different experimental methods can influence the choice of the models and this in turn will lead the mechanical properties to be found.